how many LEDs can be run off a battery
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Posts: 46,084
Hi,
This is a pretty basic question, but hopefully you will bare with me.
I am wondering how I can figure out how many LEDs I can run off a power source?
I am rusty at this stuff, so the basics of this one are eluding me.
Lets say for example that I have LEDs that are 2 V at 20mA, and a 9V battery.
How many LEDs can I power from this battery? How do you go about figuring this
out?
thank you for you time,
xoxoxoxoxoxoxoxoxoxoxoxoxoxoxoxoxoxoxoxo
.-..-. .-..-. /),/) Ned Schwartz (\,(\ .-..-. .-..-.
"-.-" "-.-" ( ';' ) 760 Irene St. ( ';' ) "-.-" "-.-"
.-..-. .-..-. (o..)o Montreal c(..c) .-..-. .-..-.
"-.-" "-.-" OO Quebec OO "-.-" "-.-"
neddothekiddo@h...
xoxoxoxoxoxoxoxoxoxoxoxoxoxoxoxoxoxoxoxo
[noparse][[/noparse]Non-text portions of this message have been removed]
This is a pretty basic question, but hopefully you will bare with me.
I am wondering how I can figure out how many LEDs I can run off a power source?
I am rusty at this stuff, so the basics of this one are eluding me.
Lets say for example that I have LEDs that are 2 V at 20mA, and a 9V battery.
How many LEDs can I power from this battery? How do you go about figuring this
out?
thank you for you time,
xoxoxoxoxoxoxoxoxoxoxoxoxoxoxoxoxoxoxoxo
.-..-. .-..-. /),/) Ned Schwartz (\,(\ .-..-. .-..-.
"-.-" "-.-" ( ';' ) 760 Irene St. ( ';' ) "-.-" "-.-"
.-..-. .-..-. (o..)o Montreal c(..c) .-..-. .-..-.
"-.-" "-.-" OO Quebec OO "-.-" "-.-"
neddothekiddo@h...
xoxoxoxoxoxoxoxoxoxoxoxoxoxoxoxoxoxoxoxo
[noparse][[/noparse]Non-text portions of this message have been removed]
Comments
neddo@s... writes:
Lets say for example that I have LEDs that are 2 V at 20mA, and a 9V battery.
How many LEDs can I power from this battery? How do you go about figuring
this out?
thank you for you time,
In a perfect world do the following:
1) See how many LEDs can be connected in series without the sum of each LED
forward voltage exceeding the voltage source.
9 volt battery, LEDs 2 volts each.
You can connect 4 in series which will "use up" 8 volts. This leaves 1 volt
"left over."
2) The left over one volt, and ohms law will be used to calculate what
resistor is needed for the series circuit to have 20 mA.
R = E/I......R = 1/0.02....R = 50 Ohms. That's it!
In reality, the battery voltage will drop. When the battery voltage drops,
the voltage across the 50 ohm current limiting ( or setting ) resistor is
reduced.
If the battery voltage is now 8.5 volts, there is now only 0.5 volts across
the 50 ohm resistor. Now the current in the circuit is set to 0.5 volts / 50
ohms = 10 mA...Leds half as bright.
To lessen the affect of losing LED brightness versus battery voltage
reduction, use only 3 LEDs in series ( 6 volts) and you will start with 2 volts
"left
over."
The resistor value that gives 20 mA with 2 volts is 100 ohms.
Now, when the battery voltage is 8.5 volts, there is now 1.5 volts across the
100 ohm resistor. Now the current in the circuit is set to 1.5 volts / 100
ohms = 15 mA, which will appear brighter than 10 mA.
One last technial point....as the current through the LED changes, so to does
the forward voltage across the LEDs, and this curve is on the LED data sheet.
It is a small factor, but worth mentioning.
[noparse][[/noparse]Non-text portions of this message have been removed]
> In a message dated 1/4/2004 5:13:05 PM Pacific Standard Time,
> neddo@s... writes:
> Lets say for example that I have LEDs that are 2 V at 20mA, and a
9V battery.
> How many LEDs can I power from this battery? How do you go about
figuring
> this out?
>
> thank you for you time,
>
>
>
> In a perfect world do the following:
>
> 1) See how many LEDs can be connected in series without the sum of
each LED
> forward voltage exceeding the voltage source.
>
> 9 volt battery, LEDs 2 volts each.
>
> You can connect 4 in series which will "use up" 8 volts. This
leaves 1 volt
> "left over."
>
> 2) The left over one volt, and ohms law will be used to calculate
what
> resistor is needed for the series circuit to have 20 mA.
>
> R = E/I......R = 1/0.02....R = 50 Ohms. That's it!
>
> In reality, the battery voltage will drop. When the battery voltage
drops,
> the voltage across the 50 ohm current limiting ( or setting )
resistor is
> reduced.
>
> If the battery voltage is now 8.5 volts, there is now only 0.5
volts across
> the 50 ohm resistor. Now the current in the circuit is set to 0.5
volts / 50
> ohms = 10 mA...Leds half as bright.
>
> To lessen the affect of losing LED brightness versus battery
voltage
> reduction, use only 3 LEDs in series ( 6 volts) and you will start
with 2 volts "left
> over."
> The resistor value that gives 20 mA with 2 volts is 100 ohms.
> Now, when the battery voltage is 8.5 volts, there is now 1.5 volts
across the
> 100 ohm resistor. Now the current in the circuit is set to 1.5
volts / 100
> ohms = 15 mA, which will appear brighter than 10 mA.
>
> One last technial point....as the current through the LED changes,
so to does
> the forward voltage across the LEDs, and this curve is on the LED
data sheet.
> It is a small factor, but worth mentioning.
>
>
> [noparse][[/noparse]Non-text portions of this message have been removed]
I would look at it from the point of view of each LED has one
resistor and the battery is 9 volts. then for really bright LED's
you might use 20mA, or for dim one's, 10mA.
100 LED's at 20mA would be 100 x 20 = 2,000 mA or 2 amps.
Then figure out how many amps the battery is good for before the
voltage drops too low for a useable rating.
If cost were not important, but battery life was, then one could
pulse the LED's to something like a 25% duty cycle and have the LED's
glow, but get almost 4 times the battey life.
Get data on the battery. 9 V alkaline ?
niMH ? NiCad ? The mA/hr ratings will reveal what the total life
would be for the quantity of LED's you might have.
Dave
davemucha@j... writes:
I would look at it from the point of view of each LED has one
resistor and the battery is 9 volts. then for really bright LED's
you might use 20mA, or for dim one's, 10mA.
100 LED's at 20mA would be 100 x 20 = 2,000 mA or 2 amps.
Then figure out how many amps the battery is good for before the
voltage drops too low for a useable rating.
If cost were not important, but battery life was, then one could
pulse the LED's to something like a 25% duty cycle and have the LED's
glow, but get almost 4 times the battey life.
Get data on the battery. 9 V alkaline ?
niMH ? NiCad ? The mA/hr ratings will reveal what the total life
would be for the quantity of LED's you might have.
Dave
For a battery operated system, and in such systems low power consumption is
usually desirable. Having only one LED in series with 9 volts is a large waste
of power.
[noparse][[/noparse]Non-text portions of this message have been removed]
I think I am getting a handle on this now. How about if I wired the LEDs in
parallel? I suppose then I would be up against the total amps of the power
source and how fast I would suck the battery dry? Is this right?
If I were to wire the LEDs parallel, assuming they are the exact same kind
of LEDs, would I be able to get away with just one resistor? Or would they
each need a resistor?
xoxoxoxoxoxoxoxoxoxoxoxoxoxoxoxoxoxoxoxo
.-..-. .-..-. /),/) Ned Schwartz (\,(\ .-..-. .-..-.
"-.-" "-.-" ( ';' ) 760 Irene St. ( ';' ) "-.-" "-.-"
.-..-. .-..-. (o..)o Montreal c(..c) .-..-. .-..-.
"-.-" "-.-" OO Quebec OO "-.-" "-.-"
neddothekiddo@h...
xoxoxoxoxoxoxoxoxoxoxoxoxoxoxoxoxoxoxoxo
Original Message
From: <smartdim@a...>
To: <basicstamps@yahoogroups.com>
Sent: Sunday, January 04, 2004 9:16 PM
Subject: Re: [noparse][[/noparse]basicstamps] Re: how many LEDs can be run off a battery
> In a message dated 1/4/2004 6:06:37 PM Pacific Standard Time,
> davemucha@j... writes:
> I would look at it from the point of view of each LED has one
> resistor and the battery is 9 volts. then for really bright LED's
> you might use 20mA, or for dim one's, 10mA.
>
> 100 LED's at 20mA would be 100 x 20 = 2,000 mA or 2 amps.
>
> Then figure out how many amps the battery is good for before the
> voltage drops too low for a useable rating.
>
> If cost were not important, but battery life was, then one could
> pulse the LED's to something like a 25% duty cycle and have the LED's
> glow, but get almost 4 times the battey life.
>
> Get data on the battery. 9 V alkaline ?
> niMH ? NiCad ? The mA/hr ratings will reveal what the total life
> would be for the quantity of LED's you might have.
>
> Dave
>
>
> For a battery operated system, and in such systems low power consumption
is
> usually desirable. Having only one LED in series with 9 volts is a large
waste
> of power.
>
>
> [noparse][[/noparse]Non-text portions of this message have been removed]
>
>
> To UNSUBSCRIBE, just send mail to:
> basicstamps-unsubscribe@yahoogroups.com
> from the same email address that you subscribed. Text in the Subject and
Body of the message will be ignored.
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>
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>
Since each LED will have a slightly different reverse bias voltage, each
will require a separate resistor. If not, then some will hog the current,
while other would be not so bright.
If you parallel a bunch of LEDs, you'll be wasting a lot of heat from all
those current limiting resistors, and yes the battery amperage is then the
determining factor....
Mike Sokol
www.modernrecording.com
mikes@m...
" One should not increase, beyond what is necessary,
the number of entities required to explain anything"...
-William of Occam-
Original Message
From: "Ned Schwartz" <neddo@s...>
To: <basicstamps@yahoogroups.com>
Sent: Tuesday, January 06, 2004 4:58 PM
Subject: Re: [noparse][[/noparse]basicstamps] Re: how many LEDs can be run off a battery
> thank you all for the replies,
>
> I think I am getting a handle on this now. How about if I wired the LEDs
in
> parallel? I suppose then I would be up against the total amps of the power
> source and how fast I would suck the battery dry? Is this right?
>
> If I were to wire the LEDs parallel, assuming they are the exact same kind
> of LEDs, would I be able to get away with just one resistor? Or would they
> each need a resistor?
>
> xoxoxoxoxoxoxoxoxoxoxoxoxoxoxoxoxoxoxoxo
> .-..-. .-..-. /),/) Ned Schwartz (\,(\ .-..-. .-..-.
> "-.-" "-.-" ( ';' ) 760 Irene St. ( ';' ) "-.-" "-.-"
> .-..-. .-..-. (o..)o Montreal c(..c) .-..-. .-..-.
> "-.-" "-.-" OO Quebec OO "-.-" "-.-"
> neddothekiddo@h...
> xoxoxoxoxoxoxoxoxoxoxoxoxoxoxoxoxoxoxoxo
>
>
Original Message
> From: <smartdim@a...>
> To: <basicstamps@yahoogroups.com>
> Sent: Sunday, January 04, 2004 9:16 PM
> Subject: Re: [noparse][[/noparse]basicstamps] Re: how many LEDs can be run off a battery
>
>
> > In a message dated 1/4/2004 6:06:37 PM Pacific Standard Time,
> > davemucha@j... writes:
> > I would look at it from the point of view of each LED has one
> > resistor and the battery is 9 volts. then for really bright LED's
> > you might use 20mA, or for dim one's, 10mA.
> >
> > 100 LED's at 20mA would be 100 x 20 = 2,000 mA or 2 amps.
> >
> > Then figure out how many amps the battery is good for before the
> > voltage drops too low for a useable rating.
> >
> > If cost were not important, but battery life was, then one could
> > pulse the LED's to something like a 25% duty cycle and have the LED's
> > glow, but get almost 4 times the battey life.
> >
> > Get data on the battery. 9 V alkaline ?
> > niMH ? NiCad ? The mA/hr ratings will reveal what the total life
> > would be for the quantity of LED's you might have.
> >
> > Dave
> >
> >
> > For a battery operated system, and in such systems low power consumption
> is
> > usually desirable. Having only one LED in series with 9 volts is a large
> waste
> > of power.
> >
> >
> > [noparse][[/noparse]Non-text portions of this message have been removed]
> >
> >
> > To UNSUBSCRIBE, just send mail to:
> > basicstamps-unsubscribe@yahoogroups.com
> > from the same email address that you subscribed. Text in the Subject
and
> Body of the message will be ignored.
> >
> >
> > Yahoo! Groups Links
> >
> > To visit your group on the web, go to:
> > http://groups.yahoo.com/group/basicstamps/
> >
> > To unsubscribe from this group, send an email to:
> > basicstamps-unsubscribe@yahoogroups.com
> >
> > Your use of Yahoo! Groups is subject to:
> > http://docs.yahoo.com/info/terms/
> >
> >
> >
>
>
>
> To UNSUBSCRIBE, just send mail to:
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